The present invention relates to an apparatus and method for analysing a material. The present invention is particularly suitable for analysing coal and for convenience the invention will hereinafter be described with reference to its application in analysing coal. However, it will be appreciated that the invention should not be considered to be restricted to the analysis of coal.
Coal is a fossil fuel source that has carbon and hydrocarbons as its main constituents. In addition, coal also contains lesser, although still significant, amounts of silicon, aluminium; iron, calcium, sodium, potassium and other elements. These species generally report to the ash after the coal has been combusted. Some coals, such as Victorian brown coals, also contain appreciable quantities of water.
It would be desirable to be able to analyse coal in two situations. The first of these involves analysis of the coal in-situ in the coal seam to assist in~ short term mine planning and to also be able to provide a more accurate estimate of the value of coal in the seam. The second application involves analysis of the coal shortly before or during combustion. This could assist in predicting the likelihood of fouling and slagging in a coal-fired boiler or combustor, thereby enabling preventative action to be taken. Fouling and slagging deposits are a major difficulty in the power generation industry and the severity of these deposits depends upon the inorganic constituents in the coal.
A number of techniques have been described that provide for coal analysis. Known analytical techniques for determining the composition of coal in a coal seam typically require the extraction of a sample or number of samples from the seam and returning the samples to a laboratory for conventional coal analysis of coal as described on page 9.4 of Perry et. al., xe2x80x9cChemical Engineer""s Handbookxe2x80x9d, 5th Edition, McGraw Hill International Book Company, 1974.
U.S. Pat. No. 4,562,044, in the name of Bohl (assigned to The Babcock and Wilcox Company) describes a method and apparatus for the on-line analysis of a coal sample. The apparatus includes four radial arms each carrying a sample cup. An indexing motor indexes each sample cup along a circular path past a filling station where the cup is filled with pulverised coal. The cup then passes to an analysing station where various chemical analyses are performed. The cup then moves to a dumping station and a cleaning station, following which the cup is again ready for filling with pulverised coal.
U.S. Pat. No. 4,841,153 in the name of Wormald (assigned to Cogent Limited) relates to an analysis system and method for analysing coal in which the coal is bombarded by neutrons to generate gamma rays. The gamma rays are detected and the composition of the coal determined therefrom.
Other detectors bombard the coal with gamma rays or x-rays. Such systems require stringent safety precautions to be taken to avoid the possibility of exposing operating staff to x-rays or gamma rays.
Another technique that has been reported as being used on a laboratory scale for coal analysis is. laser-induced breakdown spectroscopy (LIBS) or laser spark emission spectroscopy. In this technique, a high energy laser (normally pulsed) is used to vaporise and ionise a small amount of material for analysis. The vaporised material or laser-induced breakdown plasma produces strong optical emission. Spectroscopic analysis of the optical emission gives information about the properties of the material being analysed. A discussion of one technique using LIBS is given in a paper by Ottesen et. al. entitled xe2x80x9cLaser Spark Emission Spectroscopy for In-Situ, Real Time Monitoring of Pulverised Coal Particle Compositionxe2x80x9d, published by Sandia National Laboratories (No. SAND 90-8586), on behalf of the Department of Energy, printed August 1990.
Although LIBS techniques have shown promise as being suitable for coal analysis, the present inventors are not aware of the technique being applicable beyond the laboratory scale, due to difficulties which include spectral line interference, slow sampling and response times, and calibration uncertainty.
It is an object of the present invention to provide an improved apparatus and method for the analysis of material.
In a first aspect, the present invention provides an apparatus for analysing a material comprising a laser for impinging laser light onto the material to vaporise and ionise at least part of the material and to cause spectral emissions therefrom, a plurality of detection means for detecting spectral emissions from the material, each of said plurality of detection means detecting a part of the spectrum of the spectral emissions, a plurality of data collection means to collect data from said plurality of detection means on said spectral emissions whereby each of the plurality of detection means is associated with a respective data collection means, and determining means to determine the presence and/or amount of one or more elements or species in the material.
Each of the plurality of detection means may comprise a spectrometer adjusted to a part of the spectral region. Each of the spectrometers may have a CCD detector associated with the spectrometer. The CCD detector may pass information on the spectral region to a data acquisition card or a data file in a computer or memory space. This data may then be analysed to determine the presence of one or more elements or species in the material and preferably to determine the amount or concentration of the element or species in the material.
Spectrometer types suitable for use in the present invention include grating and prism spectrographs; interferometers, such as etalon and scanning interferometer types; and filters, including coloured glass or interference filter types which allow transmission or reflection of a portion of the spectrum.
Detectors other than CCD""s (charged-coupled detectors) may also be used. Other detectors that may be used in the present invention include photodiode arrays, vidicons, photomultiplier tubes and photodiodes. The person skilled in the art would readily appreciate which detector(s) should be used.
Preferably the apparatus further comprises control means for controlling firing of the laser and for controlling and synchronising operation of the plurality of detection means. The control means may include a timing circuit to fire the laser at specified times and to operate the detection means at other specified times. It is especially preferred that the control means also synchronises operation of each of the plurality of detection means such that the plurality of detection means simultaneously detect spectral emissions from the material.
In place of the timing circuit, the control means may comprise control software to control operation of the laser and the detection means.
The apparatus will also include one or more optical systems to focus the laser light on the material and to focus the spectral emissions on the plurality of detection means. The one or more optical systems may include one or more lenses, optical fibre, prisms, beam splitters or other optical components. Although suitable optical systems are required, it will be understood that the design of the optical system does not form part of the invention concept of the present invention and the person skilled in the art will be able to design a large number of suitable optical systems without requiring inventive ingenuity. Accordingly, the optical system(s) need not be discussed further.
The laser may be any laser capable of causing vaporisation and ionisation of a part of the material. Suitable lasers include solid state lasers such as the 1064 nm Nd:YAG laser, harmonic wavelengths of the Nd:YAG laser, i.e. 532 nm, 355 nm and 266 nm; gas lasers such as excimer lasers, e.g. 308 nm XeCl, or 248 nm KrF excimer lasers; carbon dioxide lasers; liquid lasers such as dye lasers; or any wavelength/frequency shifting, harmonic generation or mixing of the above. Lasers other than those specifically mentioned may also be used. The person of skill in the art will readily be able to select an appropriate laser.
The apparatus of the present invention may be suitable for analysing material in a laboratory, on a conveyor or in the ground.
The apparatus of the present invention enables high resolution of elemental fluorescence to be obtained and largely avoids or minimises spectral interferences common in known LIBS analysers. The present invention also enables detection of a large spectral range in a single laser pulse thereby greatly decreasing time for analysis. This reduced time for analysis allows the apparatus to be used as a real-time analytical tool. It also minimises sampling errors. In this regard, the apparatus can obtain an analysis of a number of elements in that portion of the material vaporised in each laser pulse. In contrast, known LIBS apparatus required sequential analysis of the material which meant that a number of laser pulses were required. Each laser pulse would vaporise a different part of the material which could lead to errors, particularly if the material being analysed has pronounced heterogeneity.
The present invention also relates to a method for analysing a material.
In a second aspect, the present invention provides a method for analysing a material comprising subjecting said material to laser light to at least partly vaporise and ionise said material to thereby cause spectral emissions, detecting said spectral emission with a plurality of detection means, each of said plurality of detection means detecting a part of the spectrum of the spectral emission, collecting data from a plurality of detection means and analysing said data to determine the presence and/or amount of one or more elements or species in the material wherein the step of collecting data from the plurality of detection means comprises passing spectral information from the plurality of detection means to data acquisition cards associated with each of the plurality of detection means.
The method may further comprise controlling operation of the laser and the plurality of detection means.